"He must have been insane, he became so different," she is quoted as saying in the psychiatric evaluation submitted to the Oslo court on Tuesday, which has been leaked to Norway's Verdens Gang newspaper.

"It is hard to believe that these things happened. It's still hard to believe."

Shortly after the 32-year-old moved back in with her that year, he began to behave erratically, she told Torgeir Husby and Synne Soerheim, the two psychiatrists who carried out the interviews.

Mrs Behring described how her son became obsessively interested in politics and history.

"He was totally beyond reason and believed all the nonsense he said," she said.

Breivik told the two psychiatrists that his mother was his "Achilles' heel", and said that he hoped she would not attend his trial.

"I just hope that my mother is not there," he told them. "She is the only one who can make me emotionally unstable. She is my Achilles' heel."

By April this year, when he was in the final stages of preparation for his attacks, he began wearing a antiseptic face mask whenever he was inside the house, fearing she would infect him.

At one point he called the family doctor, accusing his mother of infecting his sinuses, but then failed to turn up for the appointment.

He often refused to eat anything she cooked, although he sometimes sat unusually close to her on the sofa, Mrs Behring said.

Breivik was brought up by his mother, who divorced his diplomat father when he was one year old. After a falling out in his teens, Breivik had almost no contact with his father.

The conclusion that Breivik was insane surprised some psychiatrists, who had argued that the attacks were too meticulously planned to be the work of a schizophrenic.

But if supported by an expert panel from Norway's Board of Forensic Medicine, they will save Breivik from a prison sentence, leaving him facing long years of compulsory psychiatric treatment instead.

Breivik on Tuesday refused to accept that he was insane, telling Norwegian prosecutors that he was "offended".

"He said he was afraid this would happen," prosecutor Christian Hatlo, who informed Breivik of the conclusion, told journalists.

The psychiatrists warned that Mr Breivik was likely to attempt further attacks, including suicide bombings, if he was ever released.

They said that in the 36 hours of interviews they carried out, Breivik had repeatedly spoken of plans to kill Jens Stoltenberg, Norway's prime minister, and members of the of the Norwegian royal family